Stabilizer for tin-coupled elastomers

ABSTRACT

The process of this invention can be utilized to stabilize tin-coupled rubbery polymers. This process involves adding a chelating diamine or a metal salt of a cyclic alcohol to the rubbery polymer after the tin coupling has been carried out. This invention more specifically discloses a process for improving the stability of a tin-coupled rubbery polymer which comprises adding a metal salt of a cyclic alcohol to the tin-coupled rubbery polymer subsequent to the time at which the tin-coupled rubbery polymer is coupled. The use of metal salts of cyclic alcohols does not lead to recycle stream contamination because they do not co-distill with hexane or form compounds during steam-stripping which co-distill with hexane. In other words, the boiling points of these metal salts of cyclic alcohols are very high, so they do not co-distill with hexane and contaminate recycle streams. Additionally, metal salts of cyclic alcohols are considered to be environmentally safe. In fact, sodium mentholate is used as a food additive.

This application claims the benefit of U.S. Provisional Application Ser.No. 60/151,000 filed on Aug. 27, 1999.

BACKGROUND OF THE INVENTION

Tin-coupled polymers are known to provide desirable properties, such asimproved treadwear and reduced rolling resistance, when used in tiretread rubbers. Such tin-coupled rubbery polymers are typically made bycoupling the rubbery polymer with a tin coupling agent at or near theend of the polymerization used in synthesizing the rubbery polymer. Inthe coupling process, live polymer chain ends react with the tincoupling agent thereby coupling the polymer. For instance, up to fourlive chain ends can react with tin tetrachloride thereby coupling thepolymer chains together.

The coupling efficiency of the tin coupling agent is dependant on manyfactors, such as the quantity of live chain ends available for couplingand the quantity and type of polar modifier, if any, employed in thepolymerization. For instance, tin coupling agents are generally not aseffective in the presence of polar modifiers. In any case, the actualnumber of live chain ends in the rubbery polymer is difficult toquantify. As a result, there is normally unreacted tin coupling agentleft in the polymer cement after the coupling process has beencompleted.

The free tin coupling agent is then available to react with any activeprotons present in the polymer cement to form hydrochloric acid. Forexample, excess tin coupling agent can react with most hydroxyl groupcontaining polymerization shortstops or moisture from the air. The acidgenerated can then cleave the tin-carbon bonds in the tin-coupledpolymer. Undesirable polymer degradation is, of course, the result ofthe tin-carbon bonds in the rubbery polymer being cleaved. This polymerdegradation is normally evidenced by a drop in the Mooney viscosity andmolecular weight of the polymer.

SUMMARY OF THE INVENTION

This invention relates to a process for improving the stability of atin-coupled rubbery polymer which comprises adding a tertiary chelatingamine or a metal salt of a cyclic alcohol to the tin-coupled rubberypolymer subsequent to the time at which the tin-coupled rubbery polymeris coupled. Sodium mentholate is a representative example of a metalsalt of a cyclic alcohol which is preferred for utilization in theprocess of this invention.

It is normally preferred to use metal salts of cyclic alcohols in thepractice of this invention. This is because, in commercial applicationswhere recycle is required, the use of tertiary chelating amines (such assodium t-amylate) can lead to certain problems. For instance, sodiumt-amylate can react with water to form t-amyl alcohol duringsteam-stripping in the polymer finishing step. Since t-amyl alcoholforms an azeotrope with hexane, it co-distills with hexane and thuscontaminates the feed stream. The use of salts of cyclic alcohols, suchas sodium mentholate, solves the problem of recycle streamcontamination. The sodium mentholate does not co-distill with hexane orform compounds during steam-stripping which co-distill with hexane.Since the boiling points of the cyclic alcohols generated upon thehydrolysis of their metal salts are very high, they do not co-distillwith hexane and contaminate recycle streams. Additionally, such cyclicalcohols are considered to be environmentally safe. In fact, sodiummentholate and menthol (the hydrolyzed product of sodium mentholate) areused as a food additive.

DETAILED DESCRIPTION OF THE INVENTION

The process of this invention is applicable to virtually any type oftin-coupled rubbery polymer. Such tin-coupled rubbery polymers willtypically be synthesized by a solution polymerization techniqueutilizing an organolithium compound as the initiator.

Such polymerizations will normally be carried out in a hydrocarbonsolvent which can be one or more aromatic, paraffinic or cycloparaffiniccompounds. These solvents will normally contain from 4 to 10 carbonatoms per molecule and will be liquid under the conditions of thepolymerization. Some representative examples of suitable organicsolvents include pentane, isooctane, cyclohexane, methylcyclohexane,isohexane, n-heptane, n-octane, n-hexane, benzene, toluene, xylene,ethylbenzene, diethylbenzene, isobutylbenzene, petroleum ether,kerosene, petroleum spirits, petroleum naphtha, and the like, alone orin admixture.

In the solution polymerization, there will normally be from 5 to 30weight percent monomers in the polymerization medium. Suchpolymerization media are, of course, comprised of the organic solventand monomers. In most cases, it will be preferred for the polymerizationmedium to contain from 10 to 25 weight percent monomer. It is generallymore preferred for the polymerization medium to contain 15 to 20 weightpercent monomers.

The tin-coupled rubbery polymers stabilized in accordance with thisinvention can be made by the homopolymerization of a conjugated diolefinmonomer or by the copolymerization of a conjugated diolefin monomer witha vinyl aromatic monomer. It is, of course, also possible to makerubbery polymers which can be tin-coupled by polymerizing a mixture ofconjugated diolefin monomers with one or more ethylenically unsaturatedmonomers, such as vinyl aromatic monomers. The conjugated diolefinmonomers which can be utilized in the synthesis of rubbery polymerswhich can be tin-coupled and stabilized in accordance with thisinvention generally contain from 4 to 12 carbon atoms. Those containingfrom 4 to 8 carbon atoms are generally preferred for commercialpurposes. For similar reasons, 1,3-butadiene and isoprene are the mostcommonly utilized conjugated diolefin monomers. Some additionalconjugated diolefin monomers that can be utilized include2,3-dimethyl-1,3-butadiene, piperylene, 3-butyl-1,3-octadiene,2-phenyl-1,3-butadiene, and the like, alone or in admixture.

Some representative examples of ethylenically unsaturated monomers thatcan potentially be synthesized into rubbery polymers which can betin-coupled and stabilized in accordance with this invention includealkyl acrylates, such as methyl acrylate, ethyl acrylate, butylacrylate, methyl methacrylate, and the like; vinylidene monomers havingone or more terminal CH₂═CH— groups; vinyl aromatics such as styrene,α-methylstyrene, bromostyrene, chlorostyrene, fluorostyrene, and thelike; α-olefins, such as ethylene, propylene, 1-butene, and the like;vinyl halides, such as vinylbromide, chloroethane (vinylchloride),vinylfluoride, vinyliodide, 1,2-dibromoethene, 1,1-dichloroethene(vinylidene chloride), 1,2-dichloroethene, and the like; vinyl esters,such as vinyl acetate; α,β-olefinically unsaturated nitrites, such asacrylonitrile and methacrylonitrile; α,β-olefinically unsaturatedamides, such as acrylamide, N-methyl acrylamide, N,N-dimethylacrylamide,methacrylamide, and the like.

Rubbery polymers which are copolymers of one or more diene monomers withone or more other ethylenically unsaturated monomers will normallycontain from about 50 weight percent to about 99 weight percentconjugated diolefin monomers and from about 1 weight percent to about 50weight percent of the other ethylenically unsaturated monomers inaddition to the conjugated diolefin monomers. For example, copolymers ofconjugated diolefin monomers with vinylaromatic monomers, such asstyrene-butadiene rubber copolymers which contain from 50 to 95 weightpercent conjugated diolefin monomers and from 5 to 50 weight percentvinylaromatic monomers, are useful in many applications.

Vinyl aromatic monomers are probably the most important group ofethylenically unsaturated monomers which are commonly incorporated intopolydienes. Such vinyl aromatic monomers are, of course, selected so asto be copolymerizable with the conjugated diolefin monomers beingutilized. Generally, any vinyl aromatic monomer which is known topolymerize with organolithium initiators can be used. Such vinylaromatic monomers typically contain from 8 to 20 carbon atoms. Usuallythe vinyl aromatic monomer will contain from 8 to 14 carbon atoms. Themost widely used vinyl aromatic monomer is styrene. Some examples ofvinyl aromatic monomers that can be utilized include styrene,1-vinylnaphthalene, 2-vinylnaphthalene, α-methylstyrene,4-phenylstyrene, 3-methylstyrene, and the like.

Some representative examples of rubbery polymers which can betin-coupled and stabilized in accordance with this invention includepolybutadiene, polyisoprene, styrene-butadiene rubber (SBR),α-methylstyrene-butadiene rubber, α-methylstyrene-isoprene rubber,styrene-isoprene-butadiene rubber (SIBR), styrene-isoprene rubber (SIR),isoprene-butadiene rubber (IBR), α-methylstyrene-isoprene-butadienerubber and α-methylstyrene-styrene-isoprene-butadiene rubber.

The polymerizations employed in making the rubbery polymer are typicallyinitiated by adding an organolithium initiator to an organicpolymerization medium which contains the monomers. Such polymerizationcan be carried out utilizing batch, semi-continuous or continuoustechniques.

The organolithium initiators which can be employed in synthesizingtin-coupled rubbery polymers which can be stabilized by utilizing thetechnique of this invention include the monofunctional andmultifunctional types known for polymerizing the monomers describedherein. The multifunctional organolithium initiators can be eitherspecific organolithium compounds or can be multifunctional types whichare not necessarily specific compounds but rather represent reproduciblecompositions of regulable functionality.

The amount of organolithium initiator utilized will vary with themonomers being polymerized and with the molecular weight that is desiredfor the polymer being synthesized. However, as a general rule, from 0.01to 1 phm (parts per 100 parts by weight of monomer) of an organolithiuminitiator will be utilized. In most cases, from 0.01 to 0.1 phm of anorganolithium initiator will be utilized with it being preferred toutilize 0.025 to 0.07 phm of the organolithium initiator.

The choice of initiator can be governed by the degree of branching andthe degree of elasticity desired for the polymer, the nature of thefeedstock, and the like. With regard to the feedstock employed as thesource of conjugated diene, for example, the multifunctional initiatortypes generally are preferred when a low concentration diene stream isat least a portion of the feedstock, since some components present inthe unpurified low concentration diene stream may tend to react withcarbon lithium bonds to deactivate initiator activity, thusnecessitating the presence of sufficient lithium functionality in theinitiator so as to override such effects.

The multifunctional initiators which can be used include those preparedby reacting an organomonolithium compounded with a multivinylphosphineor with a multivinylsilane, such a reaction preferably being conductedin an inert diluent such as a hydrocarbon or a mixture of a hydrocarbonand a polar organic compound. The reaction between the multivinylsilaneor multivinylphosphine and the organomonolithium compound can result ina precipitate which can be solubilized, if desired, by adding asolubilizing monomer such as a conjugated diene or monovinyl aromaticcompound, after reaction of the primary components. Alternatively, thereaction can be conducted in the presence of a minor amount of thesolubilizing monomer. The relative amounts of the organomonolithiumcompound and the multivinylsilane or the multivinylphosphine preferablyshould be in the range of about 0.33 to 4 moles of organomonolithiumcompound per mole of vinyl groups present in the multivinylsilane ormultivinylphosphine employed. It should be noted that suchmultifunctional initiators are commonly used as mixtures of compoundsrather than as specific individual compounds.

Exemplary organomonolithium compounds include ethyllithium,isopropyllithium, n-butyllithium, sec-butyllithium, tert-octyllithium,n-eicosyllithium, phenyllithium, 2-naphthyllithium,4-butylphenyllithium, 4-tolyllithium, 4-phenylbutyllithium,cyclohexyllithium, and the like.

Exemplary multivinylsilane compounds include tetravinylsilane,methyltrivinylsilane, diethyldivinylsilane, di-n-dodecyldivinylsilane,cyclohexyltrivinylsilane, phenyltrivinylsilane, benzyltrivinylsilane,(3-ethylcyclohexyl) (3-n-butylphenyl)divinylsilane, and the like.

Exemplary multivinylphosphine compounds include trivinylphosphine,methyldivinylphosphine, dodecyldivinylphosphine, phenyldivinylphosphine,cyclooctyldivinylphosphine, and the like.

Other multifunctional polymerization initiators can be prepared byutilizing an organomonolithium compound, further together with amultivinylaromatic compound and either a conjugated diene ormonovinylaromatic compound or both. These ingredients can be chargedinitially, usually in the presence of a hydrocarbon or a mixture of ahydrocarbon and a polar organic compound as a diluent. Alternatively, amultifunctional polymerization initiator can be prepared in a two-stepprocess by reacting the organomonolithium compound with a conjugateddiene or monovinyl aromatic compound additive and then adding themultivinyl aromatic compound. Any of the conjugated dienes or monovinylaromatic compounds described can be employed. The ratio of conjugateddiene or monovinyl aromatic compound additive employed preferably shouldbe in the range of about 2 to 15 moles of polymerizable compound permole of organolithium compound. The amount of multivinylaromaticcompound employed preferably should be in the range of about 0.05 to 2moles per mole of organomonolithium compound.

Exemplary multivinyl aromatic compounds include 1,2-divinylbenzene,1,3-divinylbenzene, 1,4-divinylbenzene, 1,2,4-trivinylbenzene,1,3-divinylnaphthalene, 1,8-divinylnaphthalene,1,3,5-trivinylnaphthalene, 2,4-divinylbiphenyl, 3,5,4′-trivinylbiphenyl,m-diisopropenyl benzene, p-diisopropenyl benzene,1,3-divinyl-4,5,8-tributylnaphthalene, and the like. Divinyl aromatichydrocarbons containing up to 18 carbon atoms per molecule arepreferred, particularly divinylbenzene as either the ortho, meta or paraisomer and commercial divinylbenzene, which is a mixture of the threeisomers, and other compounds, such as the ethylstyrenes, also is quitesatisfactory.

Other types of multifunctional initiators can be employed such as thoseprepared by contacting a sec- or tert-organomonolithium compound with1,3-butadiene, at a ratio of about 2 to 4 moles of the organomonolithiumcompound per mole of the 1,3-butadiene, in the absence of added polarmaterial in this instance, with the contacting preferably beingconducted in an inert hydrocarbon diluent, though contacting without thediluent can be employed if desired.

Alternatively, specific organolithium compounds can be employed asinitiators, if desired, in the preparation of polymers in accordancewith the present invention. These can be represented by R(Li)x wherein Rrepresents a hydrocarbyl radical containing from 1 to 20 carbon atoms,and wherein x is an integer of 1 to 4. Exemplary organolithium compoundsare methyllithium, isopropyllithium, n-butyllithium, sec-butyllithium,tert-octyllithium, n-decyllithium, phenyllithium, 1-naphthyllithium,4-butylphenyllithium, p-tolyllithium, 4-phenylbutyllithium,cyclohexyllithium, 4-butylcyclohexyllithium, 4-cyclohexylbutyllithium,dilithiomethane, 1,4-dilithiobutane, 1,10-dilithiodecane,1,20-dilithioeicosane, 1,4-dilithiocyclohexane, 1,4-dilithio-2-butane,1,8-dilithio-3-decene, 1,2-dilithio-1,8-diphenyloctane,1,4-dilithiobenzene, 1,4-dilithionaphthalene, 9,10-dilithioanthracene,1,2-dilithio-1,2-diphenylethane, 1,3,5-trilithiopentane,1,5,15-trilithioeicosane, 1,3,5-trilithiocyclohexane,1,3,5,8-tetralithiodecane, 1,5,10,20-tetralithioeicosane,1,2,4,6-tetralithiocyclohexane, 4,4′-dilithiobiphenyl, and the like.

The polymerization temperature utilized can vary over a broad range offrom about −20° C. to about 180° C. In most cases, a temperature withinthe range of about 30° C. to about 125° C. will be utilized. It istypically most preferred for the polymerization temperature to be withinthe range of about 60° C. to about 85° C. The pressure used willnormally be sufficient to maintain a substantially liquid phase underthe conditions of the polymerization reaction.

The polymerization is conducted for a length of time sufficient topermit substantially complete polymerization of monomers. In otherwords, the polymerization is normally carried out until high conversionsare attained. The polymerization is then terminated by the addition of atin coupling agent. Tin coupling agents can normally be used in order toimprove the cold flow characteristics of the rubbery polymer and rollingresistance of tires made therefrom. Tin coupling also leads to betterprocessability and other beneficial properties.

The tin coupling agent will normally be a tin tetrahalide, such as tintetrachloride, tin tetrabromide, tin tetrafluoride or tin tetraiodide.However, tin trihalides or tin dihalides can also optionally be used. Incases where tin dihalides are utilized, a linear polymer rather than abranched polymer results. To induce a higher level of branching, tintetrahalides are normally preferred. As a general rule, tintetrachloride is most preferred.

Broadly, and exemplarily, a range of about 0.01 to 4.5 milliequivalentsof tin coupling agent is employed per 100 grams of the rubbery monomer.It is normally preferred to utilize about 0.01 to about 1.5milliequivalents of the tin coupling agent per 100 grams of monomer toobtain the desired Mooney viscosity. The larger quantities tend toresult in production of polymers containing terminally reactive groupsor insufficient coupling. One equivalent of tin coupling agent perequivalent of lithium is considered an optimum amount for maximumbranching. For instance, if a tin tetrahalide is used as the couplingagent, one mole of the tin tetrahalide would be utilized per four molesof live lithium ends. In cases where a tin trihalide is used as thecoupling agent, one mole of the tin trihalide will optimally be utilizedfor every three moles of live lithium ends. The tin coupling agent canbe added in a hydrocarbon solution, e.g., in cyclohexane, to thepolymerization admixture in the reactor with suitable mixing fordistribution and reaction.

After the tin coupling has been completed, a tertiary chelating alkyl1,2-ethylene diamine or a metal salt of a cyclic alcohol is added to thepolymer cement to stabilize the rubbery polymer. The tertiary chelatingamines which can be used are normally chelating alkyl diamines of thestructural formula:

wherein n represents an integer from 1 to about 6, wherein A representsan alkylene group containing from 1 to about 6 carbon atoms and whereinR¹, R², R³ and R⁴ can be the same or different and represent alkylgroups containing from 1 to about 6 carbon atoms. The alkylene group Ais the formula CH₂_(m) wherein m is an integer from 1 to about 6. Thealkylene group will typically contain from 1 to 4 carbon atoms (m willbe 1 to 4) and will preferably contain 2 carbon atoms. In most cases, nwill be an integer from 1 to about 3 with it being preferred for n tobe 1. It is preferred for R¹, R², R³ and R⁴ to represent alkyl groupswhich contain from 1 to 3 carbon atoms. In most cases, R¹, R², R³ and R⁴will represent methyl groups.

The tertiary chelating amines which can be employed can also be cyclictertiary chelating amines selected from the group consisting of

(1) N,N′-dialkyl piperazine which has the structural formula

(2) 1,4-diazabicyclo[2,2,2]octane which has the structural formula

(3) N,N-tetraalkyl-1,2-diaminocycloalkanes which are of the structuralformula

 wherein n is an integer from 1 to 6 and wherein R¹, R², R³ and R⁴ canbe the same or different and represent alkyl groups containing from 1 toabout 6 carbon atoms,

(4) N,N′,N″,N′″-tetraalkyl-1,4,8,11-tetraazacyclododecanes which are ofthe structure formula

 wherein R¹, R², R³ and R⁴ can be the same or different and representalkyl groups containing from 1 to about 6 carbon atoms and

(5) N,N′,N″-trialkyl-1,4,7-triazacyclononanes which are of thestructural formula

 wherein R¹, R² and R³ can be the same or different and represent alkylgroups containing from 1 to about 6 carbon atoms.

The metal salts of the cyclic alcohols that can be used will typicallybe a Group Ia metal salts. Lithium, sodium, potassium, rubidium andcesium salts are representative examples of such salts with lithium,sodium and potassium salts being preferred. Sodium salts are typicalythe most preferred. The cyclic alcohol can be mono-cyclic, bi-cyclic ortri-cyclic and can be aliphatic or aromatic. They can be substitutedwith 1 to 5 hydrocarbon moieties and can also optionally containhetero-atoms. For instance, the metal salt of the cyclic alcohol can bea metal salt of a di-alkylated cyclohexanol, such as2-isopropyl-5-methylcyclohexanol or 2-t-butyl-5-methylcyclohexanol.These salts are preferred because they are soluble in hexane. Metalsalts of disubstituted cyclohexanol are highly preferred because theyare soluble in hexane. Sodium mentholate is the most highly preferredmetal salt of a cyclic alcohol that can be employed in the practice ofthis invention. Metal salts of thymol can also be utilized. The metalsalt of the cyclic alcohol can be prepared by reacting the cyclicalcohol directly with the metal or another metal source, such as sodiumhydride, in an aliphatic or aromatic solvent.

A sufficient amount of the chelating amine or metal salt of the cyclicalcohol should be added to complex with any residual tin coupling agentremaining after completion of the coupling reaction.

In most cases, from about 0.01 phr (parts by weight per 100 parts byweight of dry rubber) to about 2 phr of the chelating alkyl 1,2-ethylenediamine or metal salt of the cyclic alcohol will be added to the polymercement to stabilize the rubbery polymer. Typically, from about 0.05 phrto about 1 phr of the chelating alkyl 1,2-ethylene diamine or metal saltof the cyclic alcohol will be added. More typically, from about 0.1 phrto about 0.6 phr of the chelating alkyl 1,2-ethylene diamine or themetal salt of the cyclic alcohol will be added to the polymer cement tostabilize the rubbery polymer.

After the polymerization, tin coupling and stabilization has beencompleted, the rubbery polymer can be recovered from the organicsolvent. The rubbery polymer can be recovered from the organic solventand residue by means such as decantation, filtration, centrification,and the like. It is often desirable to precipitate the rubbery polymerfrom the organic solvent by the addition of lower alcohols containingfrom about 1 to about 4 carbon atoms to the polymer solution. Suitablelower alcohols for precipitation of the rubber from the polymer cementinclude methanol, ethanol, isopropyl alcohol, normal-propyl alcohol andt-butyl alcohol. The utilization of lower alcohols to precipitate therubbery polymer from the polymer cement also “kills” any remainingliving polymer by inactivating lithium end groups. After the rubberypolymer is recovered from the solution, steam-stripping can be employedto reduce the level of volatile organic compounds in the rubberypolymer.

This invention is illustrated by the following examples which are merelyfor the purpose of illustration and are not to be regarded as limitingthe scope of the invention or the manner in which it can be practiced.Unless specifically indicated otherwise, parts and percentages are givenby weight.

EXAMPLE 1

In this experiment, 0.5 mmol of N,N,N′,N′-tetramethylethylenediamine(TMEDA) was added to 250 grams of tin-coupled styrene-butadiene rubber(SBR) cement having a solids content of 15 percent. The SBR cement hadbeen freshly coupled with tin tetrachloride (SnCl₄) shortly before theTMEDA was added. The fresh SBR sample had a Mooney ML-4 viscosity at100° C. of 90. Then the cement was stored at ambient temperature forfive days. The Mooney ML-4 viscosity of the SBR sample was againmeasured and was determined to be 91 after having been stored for fivedays. Thus, the Mooney ML-4 viscosity of the SBR sample did not changeappreciably during the five-day period. The fact that the Mooneyviscosity did not change significantly during storage indicates thatpolymer degradation did not occur during storage.

COMPARATIVE EXAMPLE 2

This experiment was conducted at a control. The procedure utilized inExample 1 was repeated in this experiment, except that TMEDA was notadded to the SBR cement. During the five-day storage period, the MooneyML-4 viscosity of the SBR dropped to 81. This 9-point drop in Mooneyviscosity indicates that a significant degree of polymer degradationoccurred when the TMEDA was not present in the polymer cement.

COMPARATIVE EXAMPLE 3

This experiment was conducted at a comparative example where additionaltin tetrachloride was added to the polymer cement. The procedureutilized in Example 1 was repeated in this experiment except that 0.5mmol of additional tin tetrachloride was added to the SBR cement inplace of the TMEDA. During the five-day storage period, the Mooney ML-4viscosity of the SBR dropped to 28. This 62-point drop in Mooneyviscosity shows that a large amount of polymer degradation occurs when asignificant excess of tin coupling agent is present in the polymercement.

The results of Example 1 and Comparative Examples 2 and 3 are summarizedin Table I. As can be seen, the TMEDA stabilized the SBR sample (therewas no significant change in the Mooney ML-4 of the rubber). However,significant polymer degradation occurred in the controls which were notstabilized with a tertiary chelating alkyl 1,2-ethylene diamine. Thispolymer degradation is exemplified by the large drops in the Mooney ML-4viscosities of the SBR samples.

TABLE I Example Additive ML-4 ML-4 Change 1 TMEDA 91 +1 2 none 81 −9 3SnCl₄ 28 −62

EXAMPLE 4

In this experiment, 0.5 mmol of sodium mentholate was added to 250 gramsof tin-coupled styrene-butadiene rubber (SBR) cement having a solidscontent of 15 percent. The SBR cement had been freshly coupled with tintetrachloride (SnCl₄) shortly before the sodium mentholate was added.The fresh SBR sample had a Mooney ML-4 viscosity at 100° C. of 90. Then,the cement was stored at ambient temperature for five days. The MooneyML-4 viscosity at 100° C. of the SBR sample was again measured and wasdetermined to be 90 after having been stored for five days. Thus, theMooney ML-4 viscosity at 100° C. of the SBR sample did not changeappreciably during the five-day period. The fact that the Mooneyviscosity did not change significantly during storage indicates thatpolymer degradation did not occur during storage.

Variations in the present invention are possible in light of thedescription of it provided herein. While certain representativeembodiments and details have been shown for the purpose of illustratingthe subject invention, it will be apparent to those skilled in this artthat various changes and modifications can be made therein withoutdeparting from the scope of the subject invention. It is, therefore, tobe understood that changes can be made in the particular embodimentsdescribed which will be within the full intended scope of the inventionas defined by the following appended claims.

What is claimed is:
 1. A process for improving the stability of atin-coupled rubbery polymer which comprises adding from about 0.01 phrto about 2 phr of sodium mentholate to the tin-coupled rubbery polymersubsequent to the time at which the tin-coupled rubbery polymer iscoupled.
 2. A process as specified in claim 1 wherein the rubberypolymer is selected from the group consisting of polybutadiene rubber,polyisoprene rubber, styrene-butadiene rubber, α-methylstyrene-butadienerubber, α-methylstyrene-isoprene rubber, styrene-isoprene-butadienerubber, styrene-isoprene rubber, isoprene-butadiene rubber,α-methylstyrene-isoprene-butadiene rubber andα-methylstyrene-styrene-isoprene-butadiene rubber.
 3. A process asspecified in claim 1 wherein from about 0.05 phr to about 1 phr of thesodium mentholate is added to stabilize the rubbery polymer.
 4. Aprocess as specified in claim 1 wherein from about 0.1 phr to about 0.6phr of the sodium mentholate is added to stabilize the rubbery polymer.5. A process as specified in claim 1 wherein the tin-coupled rubberypolymer is tin-coupled with tin tetrachloride.
 6. A process as specifiedin claim 1 wherein the tin-coupled rubbery polymer is tin-coupledisoprene-butadiene rubber.